1. Field of the Invention
The invention relates to telephony in general and in particular to toll restrictor circuits preventing a telephone set user from making toll telephone calls from a rotary-dial telephone set, or similar instrument providing loop disconnect outputs compatible with rotary-dial service.
2. Description of the Prior Art
By dialing ten digits, including the area code and a telephone number, a user of a telephone set in the United States can be connected to almost any other telephone set in the United States and Canada without obtaining operator assistance. Of course, the user can obtain operator assistance in completing a toll call simply by dialing the digit "0" and verbally asking the operator to complete the call. Because it is easy to use the telephone network to make expensive long-distance toll calls, it is useful for some telephone subscribers to assure that long-distance telephone calls cannot be made from certain telephone sets. This is particularly true in business offices or motels, hotels, etc. which have one trunk line to a central office of the telephone company to which are connected multiple station sets.
Telephone sets in use in the United States use two different signaling or dialing means: loop disconnect dialing apparatus, and tone dialing apparatus. Tone dialing apparatus uses frequency signaling and is not the subject of this invention and will not be discussed herein. The invention is directed for use with loop disconnect dialing apparatus.
Telephone sets with rotary dials generate dial pulses at the nominal rate of 10 pulses per second regardless of the digit dialed; the number of dial pulses generated corresponds to the digit number, e.g. one pulse is generated for the digit "1", two for "2", etc., with ten pulses being generated for the digit "0".
Toll restriction circuits are known in the prior art. Some circuits operate on the principle that the resistance of the line connected to the central office is different when a local exchange is dialed, than when a long distance call is to be made. By measuring the resistance of the line, means have been devised to block the call thereafter. U.S. Pat. Nos. 3,201,523 and 3,488,221 present circuits and equipment based on this principle, but they have inherent disadvantages because of the variation of resistances of telephone circuits due to of such factors as differences of distance to central offices different equipment in different central offices, etc.
A more straight forward approach is based on the fact that certain dialing digits must be dialed in order for a telephone user to be able to make a long distance call. By sensing those digits means can be provided for disconnecting the telephone. U.S. Pat. No. 3,566,042 discloses one such means, an electro-mechanical device for counting dial pulses and means for interrupting the line from the telephone set to the central office. An electro-mechanical approach to the toll restriction problem suffers from disadvantages of slow speed, high cost, and inherent relative accuracy.
U.S. Pat. No. 3,692,951 shows digital electronic toll restriction means in which each individual area code or access number must be individually programmed for restriction. This circuit has the inherent disadvantage of not having flexibility to restrict calls on the basis of the number of digits dialed, or capability to restrict all calls starting with "zero" or with zero among the first three digits.
For many multiple subscriber telephone situations, it is necessary to first dial "9" to obtain access to the central office. For those applications toll calls could be made by dialing 9 and then zero. Obviously, to make a valid local call requires 9 plus seven more digits, 8 digits in total. Total digit dialing in excess of eight digits would indicate that a toll call is being attempted.
For telephone sets which require no digit to be dialed to obtain central office dial tone, toll calls can be detected by detecting that a zero has been dialed as the first digit or detecting calls having more than seven digits.
Service calls, such as information "411" and emergency calls, such as "911" are usually not restricted. When the information operator hangs up, the caller could dial the operator by dialing 0, and make a long distance call, while the toll restrictor counted neither a 0 in the first three digits nor more than seven or eight digits called.
In some parts of the country, certain digits are merely "absorbed" by the central office. The digits are not used for call signaling, but merely for central office information purposes, i.e., billing etc. Hence there are situations where a user could dial the absorb digits and then 0 defeating the 0 restriction on the first digits dialed and the total number of digits dialed as indications of toll calls being made.
In addition to the dialing characteristics which must be considered in a toll restrictor circuit for general use, it is desirable that the circuit operate independently from the telephone signaling system applied at the central office. It is also desirable that the circuit make use of modern digital technology employing commercially available integrated circuits to perform functions previously performed by analog circuits and discrete digital components.
It is therefore a primary object of this invention to provide a digital, toll restrictor circuit suitable for use with loop disconnect dialing subscriber telephone stations.
It is another object of this invention to provide a toll restrictor circuit which functions independently of any telephone network d.c. signaling system or network protection device.
It is another object of this invention to provide a toll restrictor circuit which requires no special polarity and which is connected in parallel and which presents at least 1 Mohm resistance with the telephone line.
It is another object of this invention to provide a toll restrictor circuit which disregards the dialing of one or more absorb digits.
It is another object of this invention to provide a toll restrictor circuit designed with integrated circuits which can be manually programmed to provide any or all of the following features:
restriction for digit 0 dialed in the first two or the first three digits dialed;
restriction for digit 1 dialed in the first digit;
restriction for digits dialed in excess of seven or eight digits; and
restriction when delays between digits dialed are greater than a predetermined time delay.